1. Field of the Invention
The invention relates to a process and a device for eliminating rhythmic register errors in sheet-fed printing machines with multiple-size cylinders, more particularly, multiple-size sheet-conveying cylinders having at least one first and one second gripper device.
In printing machines, in particular in the case of sheet-fed printing machines with multiple-size cylinders, for example double-size or triple-size impression or transfer cylinders, it can be observed in practice that, depending upon the printing job, the composition of the paper and the printing speed, rhythmic errors occur in the sheet transfer from an upstream cylinder to a downstream cylinder, which lead to so-called "ghosting" and which are referred to hereinbelow as rhythmic register errors. The register errors are caused, for example, by differences occurring in the gripper setting and in the gripper pad height of the gripper devices as a result of production or installation tolerances of the components, and these differences can lead to the shifting of grippers, to errors during sheet transfer and to different retaining forces, as a result of which the position of the transferred sheet changes in the gripper device which accepts it. Because this change in the sheet length occurs, respectively, only at one gripper device in the case of double-size or multiple-size cylinders, i.e., cylinders having two, three or more gripper devices, the register errors caused in this way are correspondingly to be seen only on each second, third and so forth sheet. Furthermore, the aforedescribed rhythmic register errors or register changes can also be caused by pitch errors or eccentricities of the drive gear wheels of the printing machine.
Register changes of this type are eliminated in the final installation or assembly of the printing machine by a complicated and deliberate correction of the respective components.